Police killed in Abyei during south Sudan referendum
Clashes in Sudan's disputed oil-rich Abyei region have killed at least 30 people including police, reports say.
Abyei has long been seen as a potential flashpoint for renewed north-south violence as it lies on the border and is claimed by both sides.
One report said 20 policemen had been killed, but this was not confirmed.
Reports of the violence come on the second day of voting in Southern Sudan's referendum on independence.
The vote was part of a 2005 peace deal which ended decades of civil war between the mainly Muslim north and the south, where most people are Christian or follow traditional religions.
Abyei was due to hold a separate referendum on whether to join north or south Sudan but this has been postponed indefinitely because of disagreements over eligibility.
Col Philip Aguer, a Southern Sudan military spokesman, said that the Misseriya - an Arab group who move their cattle through Abyei - attacked a village on Sunday with anti-tank weapons and artillery.
He said the Misseriya were joined by fighters from the Popular Defence Forces, a militia backed by Khartoum but whose existence was banned under the 2005 peace agreement.
Col Aguer said 20 policemen serving with Abyei's joint integrated police force were killed and another 30 wounded.
Reports said clashes in the area had taken place over three days.
However, Misseriya leader Bashtal Mohammed Salem told AP news agency that 10 Misseriya herders were killed in attacks by police on Sunday.
"They want to keep us out of the area and declare independence unilaterally," he said.
The semi-nomadic Misseryia, viewed as allies of the north, say they want to vote in any referendum in Abyei, but the Dinka Ngok, seen as loyal to the south, say that would contravene the peace deal.
Analysts say the disputed area is the most likely flashpoint for north-south tensions to turn violent during the referendum.
The BBC's Will Ross in Southern Sudan said turnout on the second day of voting was not as heavy as on the first day of the week-long vote, but voters seemed just as determined.
Our correspondent, with voters in the southern village of Kotobi, says some in the queues said they were voting for friends and relatives killed by war.
The north has promised the south it will not block its plans to secede if that is the result of the vote, as widely expected.
Our correspondent says he has not yet met anyone who says they will vote for unity.
Sudanese President Omar al-Bashir has said he will respect the outcome, but warned that an independent south would face instability.
For the referendum to be valid, at least 60% of the 3.8 million registered voters must take part.
The dispute between the Misseriya and the Dinka Ngok began over grazing rights for cattle, which are central to both communities' traditions and economies.
The run-up to the vote in the south was also marred by clashes between the south Sudanese military and rebels in the oil-rich Unity state.
Southern Sudan is one of the least developed areas in the world and many of its people have long complained of mistreatment at the hands of the Khartoum government.
South Sudanese leader Salva Kiir, casting his ballot on Sunday, urged people to "be patient", in case they were not able to vote on the first day of polling.
The international community is watching the vote closely and US President Barack Obama said it represented a "new chapter in history".
The action of Sudanese leaders would help determine whether their people move "toward peace and prosperity, or slide backward into bloodshed", he said in a statement.
Last week Mr Bashir said he understood why many southerners wanted independence, but he expressed concern at how the new nation would cope.
He said southerners living in the north would not be allowed dual citizenship, and floated the idea of the two nations joining in an EU-style bloc.
The BBC's James Copnall in Khartoum says northern politicians have sent very mixed messages about the future status of southerners in the north.
One minister said sick people would not even be allowed a needle in a hospital, he says.
In poor suburbs all around Khartoum - an area known as the Black Belt - southerners have been packing up to leave. Some 120,000 southerners have already gone back to their ancestral homes in recent weeks, he says.
The official result of the referendum is not due to be announced for at least four weeks, partly because of the logistical difficulties gathering the ballot papers from across a region the size of Spain and Portugal that has few paved roads.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.